The University of British Columbia | |||
UBC-Tokyo Conference on Novel Quantum MatterPITP/AMPEL/University of Tokyo conference
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The layered organics, κ-(ET)2X, are model systems for the study of strongly correlated half-filled-band electrons. Here we present two progresses in the Mott physics through the transport and NMR studies on this family of materials. One is the Mott criticality in 2D. κ-(ET)2Cu[N(CN)2]Cl is a Mott insulator with a quite low critical pressure of Mott transition. The resistance measurements of this material under precisely controlled pressure showed that the first-order Mott transition has a critical endpoint at 40 K, where the resistive jump vanishes and critical pressure derivative of resistance is divergent. The scaling analysis of the present transport data shows that the Mott criticality belongs to an unconventional universality class, which is theoretically predicted in the marginal quantum critical regime by Imada and coworkers. The material dependence and NMR study on the Mott criticality are also presented. The other is the realization of the spin liquid and its proximity to superconductivity. The Mott insulator κ-(ET)2Cu2(CN)3 has a nearly isotropic triangular lattice and is a model system of frustrated quantum spins. The 1H and 13C NMR experiments showed no indication of magnetic ordering down to 30 mK. The spins are likely in the quantum liquid state. Under pressure, it undergoes Mott transition to the Fermi liquid which shows superconductivity at low temperatures. The pressure-temperature phase diagram and the NMR/transport results on the nature of the spin liquid and superconductivity are presented as well as the optical study by Keszmarki and Tokura, which shows an interplay between the charge and spin degrees of freedom near Mott transition. Multiferroics, the materials in which both (anti)ferromagnetism and ferroelectricity can coexist, are the prospective candidate which can potentially host the gigantic magnetoelectric (ME) effect. A useful hint to designing of the strong magnetoelectric coupling has been gained by the recent discovery of the ferroelectricity in the transverse-spiral magnets, such as perovskite manganites. The direction of the polarization can be totally determined by the clock-wise or counter-clock-wise rotation of the spin in proceeding along the spiral propagation axis, that is called the spin helicity. In those compounds, the spontaneous P can be easily controlled by an external magnetic field of specific direction, such as the generation and/or flopping of the spontaneous polarization, which may be viewed as the gigantic ME effect. Conversely, the spin helicity can be controlled by an external electric field, as demonstrated by recent polarized neutron scattering experiments. The multiferroics based on this mechanism has recently been realized also in the conical spin state of chromite spinels where the transverse spiral component coexists with the uniform magnetization component along the cone axis. In those compounds, the clamping between the magnetic and ferroelectric domains can show up, perhaps enabling the magnetic (electric) control of the ferroelectric (ferromagnetic) domains. Multiferroics with the strong ME correlation may also provide a unique arena to test new optical effects, say the magnetoelectic optical effects. This includes the so-called magnetization-induced second harmonic generation (MSHG) and the nonreciprocal dichroism dependent on the light propagation direction (but not on the light polarization), termed the optical magnetoelectric (OME) effect. The former can be applied to probe the interface magnetism as well, while the latter may have potential of producing new optical devices with nonreciprocal functions. Strategy for exploring such multiferroics as showing strong ME coupling and novel optical functions is argued in terms of the designed spin superstructure and tailor-made materials. We consider general perspective of quantum phase transitions belonging to recently found novel universality class[1]. We take examples of metamagnetic transitions and metal-insulator transitions, where a topological transition and a Z2 symmetry breaking transition merge at a terminal of the topological quantum critical line[1,2]. We discuss structure of low-energy excitations near the quantum critical terminal. If time is allowed, we show recent detailed theoretical analyses of metamagnetic transitions on several compounds.
[1] T. Misawa, Y. Yamaji and M. Imada, J. Phys. Soc. Jpn. 75 (2006)
083705, cond-mat/0612632 Transition-metal oxide thin films grown on single crystalline substrates exhibit various interesting physical properties arising from epitaxial strain from the substrates as well as from the polarity discontinuity at the film surfaces and interfaces. In particular, hole-doped perovskite-type Mn oxides are expected to show a variety of phenomena associated with the orbital degree of freedom. We have studied the chemical potential shift and orbital polarization in strained manganite thin films by photoemission and x-ray absorption spectroscopy. Compressive and tensile strain was found to raise and lower Mn-derived energy levels, respectively. Epitaxial strain was also found to suppress charge-orbital ordering through prohibiting lattice distortion. Surprisingly, the eg orbitals of Mn were found to be polarized perpendicular to the surface irrespective of the sign of the epitaxial strain and the absence or presence of SrTiO3 overlayer, suggesting a peculiar crystal field in the surface/interface layer of the manganite films, possibly arising from polarity discontinuity. This work has been done in collaboration with H. Wadati, M. Takizawa, K. Ebata, A. Chikamatsu, H. Kumigashira, A. Maniwa, M. Oshima, M. Lippmaa, M. Kawasaki, H. Koinuma, Y. Tomioka and Y. Tokura. The quantum dynamics of the magnetization in metallic ferromagnet with relativistic spin-orbit interaction is considered. For the transport properties such as the anomalous Hall effect, the band crossings near the Fermi energy, which act as magnetic monopoles in momentum space, play crucial role to determine the distribution of the k-space Berry curvature distribution. This Berry curvature is also relevant to the effective commutation relationship between the components of the magnetization. We will show that this Berry curvature can leads to some non-trivial consequences. This work has been done in collaboration with M.Onoda and A. Mishchenko. We report discovery of two geometrically frustrated transition metal oxides, Na4Ir3O8 and LiRh2O4. Na4Ir3O8, a Mott insulator with d5 (low spin state) Ir4+, crystallizes in hyper-Kagome structure, which can be viewed as a cation-ordered spinel structure. We show that the ground state of this system is a spin liquid as a consequence of geometrical frustration. A new spinel oxide, LiRh2O4, has a mixed valent cofigutarion with 1:1 mixture of low spin Rh3+ (d6) and Rh4+(d5). We found that this compound shows two sequential phase transitions at 230 K and 170 K on cooling from room temperature. The first transition is from a metal to a metal and essentially an ordering of triply degenerate t2g orbitals. The second one is from a metal to an insulator and essentially ordering of t2g holes. A drastic decrease of thermoelectric power was observed on going from orbital disordered to orbital ordered metal at 230 K, which we argue is an evidence for the enhancement of thermopower due to orbital degeneracy. The Kondo effect and superconductivity are both typical spin-correlation phenomena promoted by spin-singlet coupling. The Kondo singlet is formed for a local magnetic moment and conducting electrons at Fermi sea, while the Cooper pair singlet is formed for condensed electrons. For a quantum dot, the Kondo effect appears when an unpaired electron spin trapped by the dot is screened by the singlet coupling to conducting electrons in the tunnel-coupled contact leads, and can be suppressed by the superconductivity in the leads when the Kondo temperature TK is lower than the superconducting gap Δ. We have prepared an InAs self-assembled dot with Al contact leads to study the competition between the two kinds of singlet couplings. When TK > Δ, we observe the Kondo zero-bias anomaly for odd occupancy of the dot. In addition, we observe the first-order Andreev reflection at the bias of +Δ, strongly enhanced by the Kondo effect. On the other hand, for even occupancy of the dot, we only observe the onset of direct quasi-particle tunnelling for a bias of +2Δ. We find that that the Kondo conductance normalized by the value for the contact leads in the normal state shows a scaling behaviour with respect to the relative strength of TK/Δ. The Kondo effect observed for an InAs dot with superconducting leads, and also with normal metal leads will be discussed at the workshop. TRIUMF is the center for particle physics in Canada but it also used extensively in other areas such as condensed matter physics. In particular it is one of the world's most intense sources of polarized muons and is the North American center for muon spin spin rotation. Muons are a very sensitive probe of magnetic fields and are widely used in studies of exotic superconductivity and magnetism. Recently we have developed a closely related technique called low energy beta detected NMR where the probe is a low energy beam spin polarized 8Li. In this talk I will compare the two techniques and demonstrate with recent examples how the two techniques complement one another. The success of UBC's research in high temperature superconductivity is based on a tightly coupled mix of materials development, cutting edge experimental techniques, and theoretical and computational work. The success of this effort is further fueled by a network of collaborators at many institutions within and outside Canada. I will outline recent work at UBC on the cuprate superconductors, including the study of materials near quantum critical points and experiments using sychrotron-based techniques such as ARPES and X-ray scattering. In this talk I describe the growth and properties of the dilute bismide semiconductor alloy GaAs1-xBix and show how its properties are in certain respects complementary to the dilute nitride alloy, GaNyAs1-y. Like the dilute nitrides the dilute bismides show a giant band gap bowing effect in which a small concentration of the alloying element has a disproportionate effect on the band gap, however in the case of the bismide the band gap reduction is associated with an increase in the energy of the valence band maximum (VBM) rather than a reduction in the energy of the conduction band minimum (CBM). Under standard GaAs growth conditions Bi acts as a surfactant with associated improvements in surface quality. In order to incorporate Bi, growth temperatures below 400oC are used with As2/Ga flux ratios close to unity. The electron mobility of GaAs is only weakly affected by Bi alloying, in contrast to the dilute nitrides where the electron mobility decreases rapidly with N alloying. Bi alloying also produces a giant bowing effect in the spin orbit splitting in the valence band. Strong room temperature photoluminescence is observed. Prospects for future device applications of this new compound semiconductor alloy are discussed. Some second and third order nonlinear responses of semiconductor waveguide-based planar 2D photonic crystals will be described. Results from GaAs, InP, and Silicon-based structures demonstrate "useful" classical nonlinear responses at low absolute optical powers (microwatts and below), including sum-frequency mixing in multimode, 3D defect microcavities. Recent results on silicon microcavities that have incorporated site-selectively-attached PbSe nanocrystals to approach the quantum optics domain will also be reported. Modern science and technology rely on the utilization of material-based devices whose properties are defined by the material’s electronic structure. To achieve new electronic functionalities for the next generation of devices, the Quantum Materials Spectroscopy Center (QMSC) will be established at the Canadian Light Source for the design and exploration of novel complex materials. The QMSC includes resonant, spin- and angle-resolved photoemission spectroscopy beamlines for probing the electronic structure, and a dedicated in-situ materials preparation facility. In this talk, I will review the current state of the technique by presenting our results on unconventional oxide superconductors. I will discuss the recently discovered quasiparticle anisotropy reversal on the overdoped side of the high-Tc cuprate phase diagram and the impurity-control of valence, spin, and orbital state in ruthenium oxides. Finally, I will illustrate the characteristics which will make QMSC a premiere center at the international level for the study of novel electronic materials and structures. We report measurements of pure spin currents (spin currents without accompanying charge currents) in a GaAs 2DEG, generated and detected using quantum point contacts in a large in-plane magnetic field. We then discuss plans to perform similar experiments at low magnetic field using spin-orbit interaction instead of Zeeman energy to separate spins. Such measurements will require new materials with a stronger spin-orbit interaction, such as InGaAs or GaN heterostructures. In recent work I used the so-called momentum average (MA) approximation to obtain a very simple analytical expression for the Green’s function of a Holstein polaron. This expression is asymptotically exact for both weak and strong electron-phonon coupling, and is remarkably accurate for intermediary couplings, given its simplicity. However, it does miss some essential physics, for example the existence of a continuum at a frequency Ω above the ground-state for any coupling, or the momentum dependence of the self-energy. I will now show how MA can be systematically improved in a sequence of more and more accurate -- yet still easy to evaluate -- approximations, which I call MA(n) (the initial MA is MA(0)). MA(1) already exhibits the continuum, and MA(2) and higher levels also predict a momentum dependent self-energy, while also systematically improving agreement with the exact (numerical) results. I will show that the standard Kondo model, used to describe magnetic impurities in metals and gated semi-conductor quantum dots, predicts the appearance of a large length scale, ξK = vF/TK, which may be thought of as the extent of the wave-function of the electron which forms a spin singlet with the impurity. I will suggest experiments on mesoscopic systems that might finally lead to its observation. |